U.S. Pat. No. 4,793,191, assigned to Endress & Hausen Flowter A. G. which is incorporated herein by reference, describes a mass flow meter working on the Coriolis principle
--which is insertable into a conduit of a given diameter so as to be axially aligned with said conduit, through which flows a fluid to be measured,
--with an inlet tube and an outlet tube,
--which serve to connect the mass flow meter with the conduit,
--with an inlet manifold and an outlet manifold,
--with an external support tube
--whose ends are fixed to the inlet tube and outlet tube, respectively,
--with two annular diaphragms
--via which the inlet and outlet tubes are connected to the inlet manifold and outlet manifold, respectively,
--with two parallel, straight measuring tubes of the same inner diameter and the same wall thickness each having its two ends fixed in the inlet manifold and outlet manifold, respectively,
--with a vibration exciter
--which excites the measuring tubes into sympathetic vibrations,
--with two sensors spaced along the measuring tubes for sensing their vibrations,
--with driver electronics for the vibration exciter, and
--with evaluation electronics
--which determine the mass flow rate from the phase difference of the sensor signals and/or the density of the fluid from the vibration frequency.
For various reasons, e.g., because of the sensitivity of the mass flow meter to vibrations originating from the conduit or because of the dependence of the measurement result on the pressure of the fluid, the diaphragms in this prior art mass flow meter cannot be made arbitrarily soft but must have a given minimum stiffness.
Since changes in the temperature of the fluid result in inhomogeneous temperature distributions in the mass flow meter, they cause stress in the vibrating measuring tubes and in the diaphragms. If this stress reaches values above the yield point of the diaphragm, irreversible plastic deformation will take place, which irreversibly changes the characteristics of the vibrating system, so that recalibration of the mass flow meter will be necessary.
The arrangement described in U.S. Pat. No. 4,653,332 seeks to solve the problem just described by means of compensating tubes. This arrangement is a mass flow meter working on the Coriolis principle
--which is insertable into a conduit of a given diameter through which flows a fluid to be measured,
--with an inlet tube and an outlet tube
--which are combined to form a central connection unit and serve to provide such a connection with the conduit
--that the axis of the mass flow meter and that of the conduit do not coincide,
--with a first straight compensating tube, which is connected with the inlet tube, and a second straight compensating tube, which is connected with the outlet tube,
--the two compensating tubes extending in opposite directions and being axially aligned,
--with two straight measuring tubes which are parallel to each other and to the compensating tube such
--that the axes of said four tubes lie in one plane,
--with manifolds for respectively connecting the first and second compensating tubes with the ends of the measuring tubes,
--with a supporting tube fixed to the central connection unit,
--with a vibration exciter
--which excites the measuring tubes into sympathetic vibrations,
--with two sensors spaced along the measuring tubes for measuring the vibrations of the latter,
--with driver electronics for the vibration exciter, and
--with evaluation electronics
--which determine the mass flow rate from the phase difference of the sensor signals and/or the density of the fluid from the vibration frequency.
Since the axis of this prior art mass flow meter is not aligned with the axis of the conduit, the fluid must change its direction of flow twice by 90.degree. in the central connection unit and by 180.degree. in each of the manifolds, which may cause turbulences and cavitation problems. This mass flow meter therefore gives away the valuable advantage of the first-explained mass flow meter, in which the fluid does not change its direction of flow, since the axis of the conduit is aligned with that of the mass flow meter.
The invention as claimed serves to increase the resistance to cyclic temperature stress and the resistance to thermal shock of the first-explained mass flow meter, and thus to expand the usable temperature range of this flow meter.